Modern gas chromatographic analytical systems are particularly susceptible to performance variations due to variations in the ambient conditions in which the analytical system is operated. For example, the outlet pressure of the chromatographic column is substantially equal to the ambient atmospheric pressure, which is an uncontrollable variable. FIG. 1 shows one typical variation of ambient atmospheric (hereinafter barometric) pressure recorded over 17 days; pressure variations were found to vary over a range of approximately 4%. Additionally, analytical methods and processes are sometimes developed or performed on chromatographic systems in centrally-located analytical labs, and then transferred to other locations for continued development, research, application to production line processing, or field monitoring. Any barometric pressure differences between such locations will result in some variation in the performance of the chromatographic systems at the respective locations.
Proper analytical test methods therefore include frequent recalibration so as to correct for any systematic errors or shifts due to barometric pressure changes. Further, some conventional chromatographic equipment incorporates a form of pressure control or mass flow control for correcting an error due to ambient conditions.
In U.S. Pat. No. 4,141,237, issued to DeFord et al., a method and apparatus is disclosed wherein errors in a chromatographic analysis caused by changes in ambient atmospheric pressure are said to be corrected. In such an analysis, the output signal from a chromatograph is summed with the output signal from a pressure transducer. The output signal from the pressure transducer varies with changes in the atmospheric pressure from a reference pressure and is calibrated in such a manner as to provide a corrected chromatographic analyzer output signal when it is summed with the output signal from the chromatograph. In this manner pressure compensation is said to be provided where normalization of the chromatographic analyzer output signal is not possible or is undesirable.
In U.S. Pat. No. 4,512,181 issued to Ayers et al., errors in a chromatographic analysis caused by barometric pressure variations are said to be corrected by dividing an actual measured analysis value (Cm) by a correction factor that is calculated according to the actual atmospheric pressure at the time the measurement is made (Pa), the atmospheric pressure at which the chromatographic analyzer system was calibrated (Pc), and the slope of a plot of Cm/Ca as a function of Pa/Pc where Ca is the magnitude that Cm would have if errors were not introduced by changes in barometric pressure.
In U.S. Pat. No. 4,196,612, issued to Clardy et al., an absolute back pressure regulator is said to provide a constant reference pressure for a chromatographic analyzer system by supplying all the gaseous streams of the chromatographic analyzer system (which are normally vented to the atmosphere) to the input side of the absolute back pressure regulator. These gaseous streams include the sample vent for the chromatographic analyzer sample valve, the sample vent for the chromatographic analyzer sample detector, and the carrier vent for the chromatographic analyzer reference detector. The carrier gas pressure regulator is also said to be referenced to the constant pressure supplied by the absolute back pressure regulator instead of to atmospheric pressure. However, the column effluent and solvent venting apparently are made to pass through fluid lines to the absolute back pressure regulator. Such an approach requires another flow path that is subject to pressure leaks and other problems which may cause errors in pressure control or, another valve and pressure sensor may be needed in the split line, which adds undesirable complexity. Further, a back pressure regulator and the fluid lines that serve such a regulator may be subject to occlusion by deposits of compounds in the fluid steam, or to corrosion from the detector effluent from destructive detectors (e.g., a flame ionization detector or FID). The disclosed approach also can be subject to silent failure, wherein a failure in the reference pressure pneumatics can result in the loss of the desired control signal if, for example, the absolute pressure regulator were to become stuck open. Severe failure of the reference pressure pneumatics may occur if the same regulator were to become closed.
Accordingly, a need exists for a chromatographic system wherein the effect of barometric pressure on the passage of the carrier fluid through the chromatographic column is more accurately and reliably controlled.